Amplifiers used in small devices, for example hearing aids, typically have three stages; input, gain, and output. Battery life, size and amplifier size have long been a problem in these small products. It is desirable to have small components which will operate with low battery drain and at a low battery voltage.
Battery life and size have been restricted by the limited voltage swing capability of typical amplifiers. Usually, these amplifiers will not operate reliably below 1.5 V with ground or negative rail sensing capability, or below 1.0 V without negative ground rail sensing capability.
The typical gain stage requires a relatively large capacitor for frequency compensation. This can require the compensation capacitor to be externally connected to the amplifier integrated circuit. This increases both the size and cost of the amplifier. Additionally, the limited phase margin caused by the typical gain stage results in limited ranges of operating configurations beyond which the overall amplifier would require further external frequency compensation.
It is an object of the present invention in certain of its aspects variously to increase the common mode input voltage range and the voltage swing capability of this type of amplifier, decrease the power consumption, and increase the available phase margin of the gain stage.